The present invention generally relates to methods of supplying recording sheets, and more particularly to a method of supplying recording sheets of an image forming apparatus.
An image forming apparatus generally refers to various types of copying machines, facsimile machines and printers. Recently, there are image forming apparatuses which use a plurality of paper supplying units for supplying paper, that is, recording sheets on which images are formed. In the image forming apparatus which uses a plurality of paper supplying units, the paper supplying units are often independent and detachable from a main body of the image forming apparatus, and in most cases, a length of a transport path from the paper supplying unit to a reference position in a vicinity of a recording position in the image forming apparatus is different for each paper supplying unit. The length of the transport path will hereinafter be referred to as a transport path length.
But in general, the transport speed of the recording sheets which are transported in the transport paths is constant regardless of the transport paths even when the transport path lengths differ among the paper supplying units and even when the paper supplying speeds of the paper supplying units are different. For this reason, a time it takes for the recording sheet to reach the reference position within the main body of the image forming apparatus from the paper supplying unit becomes different among the transport paths having mutually different transport path lengths and the problems described hereunder occur.
FIGS. 1A and 1B show timings with which the recording sheets are successively supplied from the paper supplying units for explaining the conventional method of supplying the recording sheets. For the sake of convenience, it is assumed that the paper supplying units are independent and detachable from the main body of the image forming apparatus and that the recording sheets supplied from the paper supplying unit have identical sizes.
In FIGS. 1A and 1B, Ai (i=1, 2, . . .) denotes a time when the supplying of a recording sheet #i from the paper supplying unit starts, Bi denotes a time when the supplying of a next recording sheet #i+1 starts when the image formation (recording) is to be made successively on the recording sheets, Ci denotes a time when the recording on the recording sheet which is fed from the time Ai ends, and Di denotes a time when the recording sheet .pi.i which is fed from the time Ai reaches the reference position within the main body of the image forming apparatus. The transport path length between the paper supplying unit and the reference position within the main body of the image forming apparatus for the case shown in FIG. 1A is shorter than the corresponding transport path length for the case shown in FIG. 1B.
When successively recording images on the recording sheets in FIG. 1A, the first recording sheet #1 is fed from a time A1 and reaches the reference position at the time D1. The first recording sheet #1 is thereafter transported to the recording position and the recording of an image ends at the time C1. When the first recording sheet #1 reaches the reference position at the time D1, the second recording sheet #2 is fed from the time B1 (A2) which is a predetermined time after the time D1. The recording of an image on the second recording sheet #2 is carried out similarly to the first recording sheet #1. The third recording sheet #3 and recording sheets which follow are fed and subjected to the recording in a similar manner.
When successively recording images on the recording sheets in FIG. 1B, the recording sheets are fed and subjected to the recording similarly to FIG. 1A, except that a time interval between the times Ai and Di is longer than that of the case shown in FIG. 1A because the transport path length is longer for the case shown in FIG. 1B.
As may be seen from FIGS. 1A and 1B, the first recording sheet #1 reaches the reference position in the main body of the image forming apparatus at the time D1 in each of the cases shown in FIGS. 1A and 1B, but the time interval between the times A1 and D1 in FIG. 1B is longer than the time interval between the times Al and D1 in FIG. 1A because the transport path length is longer for the case shown in FIG. 1B. On the other hand, a time interval between the times Di and Bi is conventionally set constant regardless of the transport path length. For this reason, the time Ci when the recording ends in FIG. 1B is delayed with respect to the corresponding time Ci in FIG. 1A, and the delay with respect to the case shown in FIG. 1A is accumulated with the number of recording sheets which are subjected to the recording. In other words, the time interval between the times Ai and Di increases as the transport path length increases, but the recording of the image on a previous recording sheet #i-1 progresses during this time interval. As a result, when the timing with which the recording sheets are supplied is controlled by setting the time interval between the times Di and Bi to a constant value regardless of the transport path length, there are problems in that the interval between two successive recording sheets becomes large compared to the transport path length and the above described delay inevitably occurs when the transport path length is large. Therefore, the recording speed of the conventional image forming apparatus becomes slow as the transport path length becomes long.
On the other hand, when the transport path length is short, the interval between two successive recording sheets becomes short compared to the transport path length. In some cases, there is a problem in that a rear end of the leading recording sheet overlaps a front end of the trailing recording sheet of the two successive recording sheets.